nfp: parse metadata prepend before XDP runs

Calling memcpy to shift metadata out of the way for XDP to run
seems like an overkill.  The most common metadata contents are
8 bytes containing type and flow hash.  Simply parse the metadata
before we run XDP.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>